Genius footage from Sesame Street which predates ‘Star Wars IV – A New Hope.’ You can hear why he got to play the voice of Darth Vader, despite his relative youth at the time (well… he was thirty eight). He was the first well known celebrity to appear on Sesame Street in 1969 (the pilot episode) and went on to appear in several others. He apparently recorded another alphabet for Sesame Street after his Star Wars role.

From IMDB:
According to Jones, when George Lucas was trying to cast the voice of Darth Vader, his immediate idea was to cast Orson Welles. However, he felt that Welles was too well-known for the role. So instead he looked for an actor with a deep voice, “like Orson Welles” which is how he got cast.
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